1. Field of the Invention
The present invention relates to a multiplexing system and multiplexing method of tributary signals between a plurality of upstream signal paths and a plurality of downstream signal paths corresponding to the individual upstream signal paths for multiplexing a plurality of tributary signals transmitted through the plurality of signal paths to be transmitted through a single signal path, and more particularly to a multiplexing system and multiplexing method of tributary signals that can implement a higher transmission rate than Synchronous Digital Hierarchy (SDH) systems based on the virtual container multiplexing currently utilized as Network Node Interfaces (NNIs) in the ATM (Asynchronous Transfer Mode), and that can preferably be applied to the Network Node Interfaces serving as ATM network high ends such as international node interfaces using optical cables.
2. Description of Related Art
FIG. 29 is a block diagram showing a configuration of a conventional Synchronous Digital Hierarchy system based on the virtual container multiplexing. In this figure, the reference numeral 1 designates a transmitter for multiplexing a plurality of tributary signals supplied from a plurality of upstream transmission paths to be output; 2 designates a receiver for demultiplexing the multiplex signal to a plurality of tributary signals to be supplied to a plurality of downstream signal paths; and 3 designates an optical cable for transmitting the multiplex signal.
In the transmitter 1, each reference numeral 39 designates a transmission side tributary circuit having one-to-one correspondence with the upstream transmission path; 40 designates a multiplexer for multiplexing the plurality of tributary signals to create information payloads; and 41 designates a frame generating circuit for adding to the information payloads section overheads SOH including information on the destinations of the tributary signals, and for sending them to the optical cable 3. On the other hand, in the receiver 2, the reference numeral 42 designates a frame identification circuit for synchronizing and detecting frames and for decoding the section overheads SOH to be output as phase information and the like; 43 designates a demultiplexer for demultiplexing the plurality of tributary signals in accordance with the phase information to restore the tributary signals from the information payloads; and each reference numeral 44 designates a receiving side tributary circuit to which the demultiplexer distributes the tributary signals.
FIG. 30 is a diagram illustrating a structure of an STM-N (Synchronous Transport Module-N) frame adopted by the conventional Synchronous Digital Hierarchy system. The STM-N frame was recommended as a multiplexing structure of N AUGs (Administrative Unit Groups) by ITU-T Recommendations G.707-709 (FIG. 3-1/JT-G709 in Network Node Transmission System (Ver. 6) of “Network Node Interface”, Part 1, Vol. 3 of the Telecommunication Technology Committee (TTC) Standard). In this figure, each reference numeral 45 designates an administrative unit group AUG as a tributary signal; and 46 designates an STM-N frame. The reference numeral 47 designates a information payload; 48 designates a section overhead SOH; and 49 designates an AU pointer. The recommendation specifies that the data in a plurality of administrative unit groups AUGs should undergo interleaving.
Next, the operation of the conventional system will be described.
When a plurality of tributary signals are input to the plurality of transmission side tributary circuits 39, 39 while the frame generating circuit 41 and frame identification circuit 42 are transmitting and receiving multiplex signal frames at every specified fixed interval, the multiplexer 40 multiplexes the plurality of tributary signals to generate the information payloads, and the frame generating circuit 41 adds the section overheads SOHs to the information payloads individually to create frames, and transmits them.
Receiving the frames, the frame identification circuit 42 supplies the information payloads to the demultiplexer 43, and at the same time decodes the section overhead (SOH) areas to generate phase information. The demultiplexer 43 demultiplexes the information payloads on the basis of the phase information to restore the plurality of tributary signals, and supplies the tributary signals to the receiving side tributary circuits 44, . . . , 44 designated by the phase information. The receiving side tributary circuits 44, . . . , 44 send the tributary signals to the downstream signal paths.
In this way, the conventional Synchronous Digital Hierarchy system can transmits tributary signals from the upstream signal paths to the corresponding downstream signal paths without fail.
With such a configuration, the conventional Synchronous Digital Hierarchy system must generate the information payloads by multiplexing the plurality of tributary signals, and then generate the section overheads (SOHs) containing the AU pointers to be added to the information payloads. In addition, since the multiplex signals thus generated are subjected to the interleaving before transmitted to the main signal path, the system must carry out the processing from generating the section overheads (SOHs) to adding them to the information payloads at a very high speed.
As a result, the Network Node Interface, which serves as a high end of an ATM network such as an international node interface using the optical cable 3, and which must transmit high-rate, large-volume information, presents a problem in that it is very difficult for the system to implement the multiplexer 40 or demultiplexer 43 because of the high bit-rate tributary signals themselves and of multiplexing multiple high bit-rate tributary signals, and in that even if they can be implemented, a great deal of power is consumed. In other words, making full use of the transmission rate of the optical cable 3 is limited by the operation rate of the multiplexer 40 or demultiplexer 43, which presents a problem in that the transmission cannot be implemented at the rate the optical cable 3 originally provides.